The present invention relates to an overhead projector comprising a transparent picture plate illuminated from below on which a transparent document with more or less opaque information can be supported and an optics arm which supports a projector head with optics and an oblique mirror to deflect a light beam from the picture plate towards a projection screen and to project information onto it.
When no document has been placed on the picture plate of an overhead projector, the whole surface of the picture plate is projected as a large, strongly illuminated square on the projection screen. As soon as a document is placed on the picture plate, the illumination is reduced, partly because the document itself absorbs some light and partly because the information in the form of text, pictures and/or diagrams absorbs light. Of the original illumination, there remains a square frame around the document and out to the edges of the picture plate, since the documents used are normally smaller than the picture plate. Both the light variations between successive projections and the strongly illuminated frame surrounding a projected picture disturb the audience.
In order to remedy these inconveniences to some extent, opaque rulers can be placed on the picture plate around the document. These are not used very often, since it is tiresome to have to check that the rulers are placed correctly after each change of picture.
Another method is to encapsulate each document between two light-polarizing plastic films with a size that is at least equal to the surface of the picture plate, the polarization directions being arranged so that they are at an angle of 90xc2x0 to each other. Such an arrangement means that the light, after being polarized by the film, becomes depolarized after passing the document and can thereby pass the second film without being absorbed.
In the regions around the document, however, the light must pass through two polarizing filters with polarization directions at an angle of 90xc2x0 to each other and they consequently absorb the light almost completely. No strongly illuminated frame around the projected picture is therefore created. Polarizing film is unfortunately very expensive, and this solution is therefore seldom used. Another very important disadvantage in practice is that the document with its initial information is not accessible during the display for complementary information to be drawn directly onto the document.
To be able to draw on the document is often desirable in order to clarify a process, and the document is then discarded and replaced with a new document with the initial information in a subsequent display. It would be unsuitable to draw on the upper side of the film since this would damage the film and the slightest displacement between film and document would make the information difficult to interpret. It would also be necessary to discard at least the upper polarizing film or both the document and both films, if these are sealed around the edges, after each display.
The object of the present invention is to provide a solution to the above described problems, not only so that the strongly illuminated frame around the projected picture is eliminated but also so that the illumination on the projection screen is automatically strongly reduced when the pictures are being changed. It shall also be possible to draw on the document during the display.
According to the invention, this is made possible by equipping the projector with two polarizing layers positioned essentially perpendicular to the light beam fitted at a distance from each other. Namely, a first polarizing layer covers the picture plate and is a supporting surface for the document, and a second polarizing layer is fixed in a holder in the path of the light beam close to the projector head, such that the polarization directions of the polarizing layers are arranged to be perpendicular to each other, so that if there is no document on the supporting surface the projection screen is not illuminated, whereas if there is a document on the picture plate, the transmission of light from those parts of the picture plate which are located between the edges of the document and the edges of the picture plate is counteracted by the polarizing layers and only the document is shown illuminated on the projection screen. The layers are suitably arranged on a glass plate to resist the influence of heat but can also be arranged on a heat-resistant film of a plastic material.